Metal-insertion reactions

Another chemical approach to the chemical conversion of methane involves organometallic reactions.85-89 Interesting work with iridium complexes and other transition metal insertion reactions (rhodium, osmium, rhenium, etc.) were carried out. Even iron organometallics were studied. These reactions take place in the coordination spheres of the metal complexes, but so far the reactions are stoichiometric and noncatalytic.77 In terms of synthetic hydrocarbon chemistry, these conversions are thus not yet practical, but eventually it is expected that catalytic reactions will be achieved. 

The usual products obtained from metal insertion reactions are shown in Table 3. They are used as starting materials for any axial ligand substitution processes. 

Perfluoroalkylcopper reagents are the most studied perfluoroalkyl organometallic reagents due to their unique combination of thermal stability and chemical reactivity. They are readily prepared by copper metal insertion reactions with perfluoroalkyl halides in a coordinating solvent (e.g., formation of decarboxylation reactions of perfluoroalkanoic acid salts and copper(I) halide (e.g., formation of 2 ), or metathesis reactions (e.g., formation of 4 via 3 ). (Trifluoro-methyl)copper (4) has also been recently accessed by decomposition of methyl 2,2-difluoro-2-(fluorosulfonyl)acetate (5) or methyl perfluoro [2-(fluorosulfonyl)ethoxy]acetate (6) in the presence of copper(I) iodide and by reaction of trimethyl(trifluoromethyl)silane (7) with fluoride and copper(I) iodide. ... 

Metal insertion reactions of alkyl halides produce organometal-lic derivatives such as Grignard reagents, which are synthetically useful sources of carbanions and change the reactive character of the carbon atom of the alkyl halide. 

An aryl halide such as chlorobenzene is relatively unreactive towards nucleophilic substitution. The S l and Sj. 2 pathways involve mechanisms that are not open to aryl halides. The greater s character of the sp bond makes it more difficult to cleave the bond to generate a carbo-cation. However, these restrictions do not apply to radical or carbanion chemistry. Hence, aryl halides undergo radical coupling reactions and metal insertion reactions, leading to organometallic compounds. 

Aryl and alkenyl halides undergo reactions with metals such as Zn, Mg, and Li to give products where the C-X bond is replaced with a C-metal bond. The best-known metal insertion reaction is the Grignard reaction, which uses Mg. Lithiation requires two equivalents of Li, because each Li supplies only one electron, but the Grignard and zinc insertion reactions require only one equivalent. The rate of insertion is strongly dependent on X, with I > Br 5S> Cl, corresponding to the strength of the C-X bonds. 

Alkyl chlorides, bromides, and iodides undergo metal insertion reactions with Li, Mg, and Zn, just as aryl and alkenyl halides do. The reaction is more facile for heavier halogens. 

This metal insertion reaction is nucleophilic with respect to the zerovalent metals and takes place exclusively at the cyclopropane C(l)-C(2) bond (see Table 3). 

The polysulfide species [Mo=S(S4)2] (20 in Scheme 2.19) was reported to react with an alkyne producing a metal dithiolene unit in situ. The metal insertion reaction proceeds via electrophilic attack by dimethylacetylene-carboxylate (DMAC). It was observed that replacing the sulfido ligand with an 0X0 makes the tetrasulfide ligand less reactive towards the electrophilic attack by DMAC and changes the product of the reaction of DMAC and complexes possessing a Mo(S4) as shown in Scheme 2.19. 

Metal-insertion reactions form the basis for a variety of organic reactions that are metal catalyzed. These are discussed below. 

Metal-insertion reactions in organometallic synthesis, as one of the results of metal activation, have been comprehensively reviewed by Luche and co-workers.In this section, the focus is on main group metals. 

Contents Comparison of synthetic reactions by transition metal complexes with those by Grignard reagents. -Formation of a-bond involving transition metals. -Reactivities of a-bonds involving transition metals. -Insertion reactions. - Liberation of organic compounds from the a-bonded complexes. - Cyclization reactions, and related reactions. - Concluding remarks. 

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